1. Field of the Invention
This invention relates to truss structures and, more particularly, to collapsible truss structures desirable for space applications.
2. Description of the Related Art
Truss structures are desirable in space applications because they offer high strength and/or stiffness and low mass. External loads on trusses are reacted to by the truss members in pure tension or columnar compression. Such highly directed loading allows the use of uni-directional filamentary composite tubes as members, which have very high ratios of material stiffness-to-mass. The attendant truss property of repeating symmetry makes efficient packaging possible.
The evolution of deployable space trusses has been toward greater performance in terms of high stiffness and low mass. For example, a deployable lattice column is described in U.S. Pat. No. 3,486,279 to James E. Webb. FIG. 7 thereof discloses a truss structure in which the longerons are comprised of continuous flexible rods. Thus, there are no hinges along the longerons and the full length of the longeron is utilized in terms of resisting collapse of the column, since the longerons maintain a tendency to spring back to a straight shape. The absence of longeron hinges results in a structure of low mass. The column, however, is limited in cross section because of strain in the packaged or collapsed condition. Hinging along the longerons would allow greater freedom in longeron member size, but hinge compliance would degrade member stiffness. Similar comments apply to U.S. Pat. No. 4,532,742 to Koryo Miura, which discloses a similar structure employing continuous, flexible rods as longerons but adds spacers 2 along the longitudinal axis of the structure.
The development of single-degree-of-freedom hinge technology has significantly reduced hinge compliance, and articulated structures have become viable. The structure disclosed in U.S. Pat. No. 4,480,415 to Peter Truss, in which the longerons are arranged to form a triangular cross-section, includes longeron members that are formed from pairs of foldable rod members. Pressure directed at a longeron along the triangular face collapses the longeron. Disclosed in FIG. 2 of the above-referenced patent to Webb is a deployable column in which the longerons do not fold, but in which the diagonals do collapse.
Although single-degree-of-freedom hinges reduce hinge compliance, such hinges also introduce a problem: in general, packaging a truss structure using such hinges involves member strain during the transition from fully deployed to fully collapsed, even though either extreme is unstrained. As a result, force is required for deployment which depends on the degree of strain, and member sizing is limited to that which can survive the strain. Thus, there is a continuing need for improvements in truss structures for space applications that overcome the problems mentioned above.